This patent specification relates to the field of optical fibers. More particularly, it relates to an optical fiber having a strongly negative dispersion characteristic that is advantageous for use as a dispersion compensating fiber.
As the world""s need for communication capacity continues to increase, the use of optical signals to transfer large amounts of information has become increasingly favored over other schemes such as those using twisted copper wires, coaxial cables, or microwave links. Optical communication systems use optical signals to carry information at high speeds over an optical path such as an optical fiber. Optical fiber communication systems are generally immune to electromagnetic interference effects, unlike the other schemes listed above. Furthermore, the silica glass fibers used in fiber optic communication systems are lightweight, comparatively low cost, and are able to carry tens, hundreds, and even thousands of gigabits per second across substantial distances.
A conventional optical fiber is essentially an optical waveguide having an inner core and an outer cladding, the cladding having a lower index of refraction than the core. Because of the difference in refractive indices, the optical fiber is capable of confining light that is axially introduced into the core and transmitting that light over a substantial distance. Conventional optical fibers have a solid cross-section and are made of fused silica, with the core region and the cladding region having different levels of dopants (introduced impurities) to result in the different indices of refraction. The cladding is usually doped to have a refractive index that ranges from 0.1% (single mode fibers) to 2% (multi-mode fibers) less than the refractive index of the core, which itself usually has a nominal refractive index of 1.47.
As known in the art, single-mode fiber is preferred over multi-mode fiber for high-capacity, long-distance optical communications. Single-mode fiber prevents electromagnetic waves from traveling down in the fiber in anything but a single, tightly held mode near its center axis. This is in contrast to multi-mode fiber, in which incident electromagnetic waves may travel down the fiber over several paths of differing distances. Accordingly, single-mode fiber allows for reduced group delay, and thereby allows optical signals to better keep their shape as they travel down the fiber. Conventional single-mode fibers have a core diameter of about 9 xcexcm and a cladding diameter of about 125 xcexcm, and are single-mode down to a cutoff wavelength of about 1100 nm, below which they become multi-mode.
Chromatic dispersion is one of the adverse effects suffered by conventional single-mode optical fibers. Dispersion is generally a loss of signal shape as different component wavelengths travel down the fiber at different speeds. Dispersion effects serve to reduce the rate at which a light beam at a given wavelength xcexi may be modulated with information (thereby reducing system throughput), and to reduce the required spacing between regenerators in a fiber optic communications link (thereby increasing system cost).
FIG. 1 shows a dispersion characteristic 100 for a conventional single-mode silica glass optical fiber versus wavelength, for wavelengths between 1300 nm and 1600 nmn. See Jopson, B., xe2x80x9cChromatic Dispersion Compensation and Measurement,xe2x80x9d Optical Fiber Communication Conference 2000 Proceedings, Baltimore, Md., TuC, pp. 1-28, (2000), which is incorporated by reference herein. Dispersion is commonly expressed in ps/(nm-km) and, as shown in FIG. 3, varies with wavelength. Where the dispersion is positive, longer component wavelengths tend to fall behind the shorter component wavelengths as a signal travels down the fiber. Where the dispersion is negative, shorter component wavelengths tend to fall behind the longer ones. As shown in FIG. 1, dispersion is positive in the commonly-used wavelength range of 1500 nm-1600 nm, ranging from approximately 14 ps/(nm-km) at 1500 nm to 20 ps/(nm-km) at 1600 nm.
As described in Hecht, supra at pp. 96-97, the usefulness of dispersion-compensating fibers arises out of the principle that fibers with opposite signs of chromatic dispersion can be serially combined to yield low overall dispersion. Dispersion-compensating fibers are characterized by a strongly negative dispersion characteristic. For optical fiber links operating in the 1500-1600 nm range, where the conventional fiber optic span dispersion is positive, these dispersion-compensating fibers are placed in series with the fiber span at regular intervals. As used herein, the term xe2x80x9cfiber spanxe2x80x9d shall denote those portions of conventional single-mode optical fiber between which the dispersion-compensating fiber loops are interposed. Generally speaking, after a signal passes through a fiber span having positive dispersion in which the longer wavelengths fall behind the shorter ones, the dispersion-compensating fiber section causes the longer wavelengths to xe2x80x9ccatch upxe2x80x9d with the shorter ones to cancel the dispersion effect.
One commonly used dispersion-compensating fiber comprises a single narrow core (about 2.2 xcexcm) that is heavily doped in the positive index direction (xcex94n=+4%), surrounded by a cladding region that is more lightly doped in the negative index direction (xcex94n=xe2x88x920.7%). See Jopson, supra at p. 25. However, conventional dispersion compensating fibers have dispersions of only about xe2x88x92100 ps/(nm-km) to xe2x88x92200 ps(nm-km), and therefore a substantial amount of dispersion compensating fiber is required per unit distance of optical fiber span. Thus, for example, for every 100 km of fiber span having a dispersion of +20 ps/km-nm, there would need to be 20 km of dispersion-compensating fiber having a dispersion of xe2x88x92100 ps/km-nm. The need to place a 20 km loop of dispersion-compensating fiber for every 100 km of fiber span introduces unwanted costs in the construction of a fiber optics communications link.
Disadvantageously, because dispersion compensating fibers are optimized for dispersion rather than attenuation, they typically have substantial attenuations of about 0.5 dB/km. Thus, the 20 km dispersion compensating fiber of the above example introduces a large amount, about 10 dB, of attenuation. Further, this additional attenuation is often compensated for by increasing the amplification of optical amplifiers in the communications link, which increases noise in the transmitted signal. Additionally, the dispersion characteristic of conventional dispersion compensating fibers tends to be rather flat between 1500 and 1600 xcexcm, whereas a more effective dispersion compensating fiber would have a dispersion characteristic that is closer to a scaled negative mirror image of the dispersion characteristic of the fiber span. Thus, for example, where the fiber span has a roughly linear increase from 14 ps/(nm-km) to 20 ps/(nm-km) (about a 43% increase) between 1500 nm and 1600 nm, an effective dispersion compensating fiber would have a concomitant decrease of 43% (i.e., becomes 43% more negative) across this same wavelength range.
Proposals have been made for dispersion compensating fibers having stronger negative dispersion characteristics. One proposal is described in Thyagarajan et al, xe2x80x9cA Novel Design of a Dispersion Compensating Fiber,xe2x80x9d IEEE Photonics Technology Letters, Vol. 8, No. 11, (November 1996) (hereinafter xe2x80x9cThyagarajanxe2x80x9d), which is incorporated by reference herein. The Thyagarajan fiber comprises a dual-core design comprising an inner core of radius a and refractive index n1, an inner cladding surrounding the inner core having an outer radius b and refractive index n3, an outer core surrounding the inner core having an outer radius c and refractive index n2, and an outer cladding surrounding the outer core having a refractive index of n3 and extending out to the rest of the radius of the fiber. The fiber is characterized as highly asymmetric in that the refractive index difference between the inner core and the cladding (about 2%) is many times greater than the refractive index difference between the outer core and the cladding (about 0.2%).
While the Thyagarajan fiber is capable of reaching a very strong negative dispersion of about xe2x88x925000 ps/(nm-km), it only reaches this value for a very small band of wavelengths about 5-10 nm wide. One form of the Thyagarajan fiber, for example, reaches xe2x88x925000 ps/(nm-km) at 1.55 xcexcm, but is only xe2x88x92500 ps/(nm-km) at the immediately neighboring wavelengths of 1.54 xcexcm and 1.56 xcexcm. This notch-like characteristic makes the fiber less useful in practical applications, where a more consistent dispersion characteristic approximating a scaled negative mirror image of the fiber span dispersion characteristic is needed.
Accordingly, it would be desirable to provide an optical fiber having a strongly negative dispersion characteristic across a substantial range of wavelengths.
It would be further desirable to provide an optical fiber have a dispersion characteristic that is closer to a negative mirror of the dispersion characteristic of a conventional single-mode optical fiber, for providing enhanced dispersion compensation of signals propagated along a conventional single-mode optical fiber span.
It would be even further desirable to provide a dispersion compensating fiber having a shorter required length per unit distance of optical fiber span, for lowering system costs and reducing attenuation caused by dispersion compensating fiber.
In accordance with a preferred embodiment, an optical fiber is provided comprising a primary core, a primary cladding surrounding the primary core, a secondary core surrounding the primary cladding, and a secondary cladding surrounding the secondary core, wherein a plurality of voids is formed within the secondary core. The voids in the secondary core are elongated and run parallel to the fiber axis, and form a spatially periodic pattern when viewed in cross-section. The spatially periodic void pattern is characterized by at least one pitch parameter representing distances between each void and its neighbors. According to a preferred embodiment, the pitch parameter is greater than or equal to approximately one wavelength of light being propagated, and each void has a dimension that is substantially less than the pitch parameter. In one preferred embodiment, the voids are circular and the spatially periodic void pattern comprises hexagonal unit cells having central members, whereby each void is the same distance from each of its six nearest neighbors. Preferably, the refractive index difference between the primary core and the primary cladding is several times greater than the refractive index difference between the secondary core and the primary cladding. The secondary cladding has an index of refraction substantially equal to that of the primary cladding.
An optical fiber in accordance with the preferred embodiments exhibits a strongly negative dispersion characteristic, thereby reducing the required length of dispersion compensating fiber per unit distance of optical fiber span. This lowers system costs and reduces attenuation caused by the dispersion compensating fiber. Furthermore, the dispersion characteristic more closely approximates a scaled negative mirror image of the dispersion characteristic of a conventional fiber span, thereby providing improved dispersion compensation performance.